Optical disk assemblies of the kind described above are useful for storing information in the form of minute, optically-readable deformations which are spaced along spiral or concentric tracks on a disk-shaped storage medium. Such assemblies offer the advantage of increased information capacity over previously used information-storage media such as magnetic tape and microfilm. In general, use of an optical disk recording medium involves forming micrometer-sized information bits (e.g. light-modulating discontinuities) along recording tracks on the surface of the disk. One common way to form such discontinuities in real-time (i.e. So they are readable without an intermediate processing procedure) is to scan the record surface of a disk with a focused beam of radiation (e.g. from a laser) which is turned on and off according to an electrical signal representative of the information to be recorded. The beam thus forms tracks of discrete deformations in the disk record surface. On playback, the tracks are illuminated by a tightly focused reading beam whereby variations in the radiation reflected from deformed and nondeformed track portions are sensed by a photodetector to reproduce the electrical signal.
Although the disk configuration described above is currently the most popular format for such optical storage media, it should be noted that there are useful formats other than the disk. For convenience, the discussion of this invention will refer primarily to the recording elements as optical disk assemblies, with the understanding that the present invention could be utilized in conjunction with equivalent elements.
A troublesome problem with optical disks is the difficulty of maintaining the recording tracks in a highly precise configuration (e.g. concentric circles or convoluting spiral track). This difficulty is more prominent when recording elements are destined for storage and/or use under a wide variety of environmental conditions (i.e. ranging from frigid to tropical temperatures, and arid to humid conditions) over long periods of time (i.e. for archival storage). Such conditions may cause the configurations to change because of dimensional changes in the disk support due to tension loss.
In order to write and read information in the form of the minute markings described hereinabove, optical systems of high numerical aperture are used to focus light to equivalently minute spots. Such optical systems have extremely small depths of focus and the proper positional relationship between the writing or reading optical system and the optical disk record surface must be stringently maintained. Therefore it is highly desirable that the record layer, and thus the optical disk support surface underlying the layer, be smooth and flat. By smooth it is meant that the surface is relatively free of high-spatial-frequency variations from a nominal plane, e.g. such as caused by minute pits or bumps. By flat it is meant that the surface is relatively free of large amplitude, low-spatial-frequency variations, such as caused by undulating surface variation or sags caused by tension loss. Although complex devices can be used to compensate for imperfect smoothness and flatness, these devices are quite expensive and add complication to the whole write/read system.
One approach to achieve requisite smoothness and flatness has been to form the disk support from a rigid material, such as glass with a ground and polished surface. This approach, however, involves time-consuming and costly fabrication procedures. Another approach is to form a plastic disk support with a highly finished surface out of a mold and to apply to the support a surface smoothing sub-layer. However, it is extremely difficult to mold such plastic disks having adequate surface characteristics at high production rates, as this support fabrication method is usually tedious (i.e. piecemeal manufactured).
Still another approach for meeting smoothness and flatness requirements is disclosed in F. F. Geyer and E. M. Leonard, U.S. Pat. No. 4,365,258 issued Dec. 21, 1982. In that approach, an improved optical disk assembly adapted for high density storage of information comprises (i) a flexible, disk-shaped polymeric support web carrying a record layer; (ii) a transparent disk cover sheet opposing the record layer, and (iii) an annular retaining ring for holding the support web and cover sheet, collectively referred to as the web assembly, in a relatively low circumferentially-symmetric tension. The flexible polymeric support materials described as useful include polyester films, such as biaxially-oriented poly(ethylene terephthalate) films.
However, it is well known that most flexible polymeric support materials, such as poly(ethylene terephthalate), are highly responsive to environmental conditions (e.g. temperature, humidity, etc.) during manufacture or use. Further, such materials are subjected to considerable mechanical stresses during manufacture. Such stresses often cause gradual changes to subsequently occur in the materials over a long period of time resulting in dimensional instability. such instability can cause support sag and localized stresses. Efforts are ongoing in industries making and using such materials in film form to improve their dimensional stability, particularly when they are subjected to elevated temperatures.
For instance, in the photographic industry, films of polyesters, e.g. poly(ethylene terephthalate), are often used as supports for various imaging elements, particularly those elements which are subjected to elevated temperatures during either processing or use. Yet, such polyester supports tend to distort in either or both planar dimensions under such conditions. As a result, various manufacturing operations have been devised to improve dimensional stability and to minimize such distortions. Such operations are known in the art as "heat-setting" and "heat-relaxation" operations as described, for example, in U.S. Pat. Nos. 2,779,684 (issued Jan. 29, 1957 to Alles) and 4,076,532 (issued Feb. 28, 1978 to Gottermeier). Generally, such operations involve heating the polymeric film during manufacture, while it is under tension in one or both planar dimensions, at temperatures above the glass transition temperature (Tg) of the polymer for short periods of time.
For some uses of such polymeric films, "heat-setting" and "heat-relaxation" operations do not provide sufficient dimensional stability. Therefore, additional treatment is carried out in some instances. Exemplary additional heat treatments are described in Research Disclosure, publication 19809, October, 1980 (published by Industrial Opportunities, Ltd., Homewell, Havant Hampshire PO9 1EF United Kingdom). The treatments described therein generally involve heating the "heat-seat" and "heat-relaxed" film to elevated temperatures (usually greater than the Tg) under various tensioning conditions.
Another polymeric film treatment following "heat-setting" and "heat-relaxation" is described in U.S. Pat. No. 4,141,735 (issued Feb. 27, 1979 to Schrader and Carroll, Jr.). The process described therein involves heating a sheet or roll of polymeric film under certain time and temperature conditions to reduce the tendency of that film to develop core-set curl when stored or used in roll form wound around a core or spool. In effect, this treatment reduces the free volume of the polymeric material so that it reaches a level of stabilization against forming core-set curl faster than it would if left to itself.
Maintaining track configuration, and surface flatness and smoothness in optical disks has proven difficult when flexible polymeric materials are used therein in film form. The dimensional stability of such materials is generally too low for optical disks where distortions on the order of a few micrometers can make the disk worthless. The additional heat treatments devised for treating polymeric films used in the photographic art, described in the references noted hereinabove, do improve dimensional stability to a degree, but insufficiently to reach the critical level of stability required for optical disk supports.
Hence, there is a need in the art for a process for making thermoplastic polymeric materials highly dimensionally stable so that they can be used as support materials in recording elements such as optical disk assemblies.